An oil-like substance has been bubbling up in the River Avon in Bristol for years and killing wildlife, according to local residents who have made regular reports to the Environment Agency (EA).
Nicola Irvine, who lives in Crews Hole, said it had been like "pulling teeth" getting the EA to investigate the pollution since she contacted them in July 2025.
Her partner, Toby White, said: "It's hard to describe how bad the smell is - at times it's like you're living on a petrol station and you start getting headaches."
The EA said it had already visited the site and was "continuing to investigate the source of contamination and its damage to the environment".
An EA spokesperson added: "While impacts have been found where the contamination has entered the watercourse, we have not found wider environmental effects downstream.
"We continue to work with landowners, the local authority and partner organisations to take action to protect the local environment."
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Nearby is a popular swimming spot at Conham River Park where there has been years of campaigning to protect the Avon from pollution, most recently with a charter of rights for the river.
Irvine said she first noticed the oily substance leaving a rainbow film on the river's surface and a strong acrid smell in early 2025.
She reported it to the EA in July that year when she believed it was bubbling up rather than coming from boats.
The EA first dismissed the substance as surface run-off but, after Irvine continued sending photos, including of dead wildlife, the investigation reopened in May this year.
"This stretch of the Avon used to regularly have 30 to 40 ducks, but now it's unusual to see more than a handful," she said.
"I have personally found dead fish, ducks and ducklings. I know we can't prove it, but there has to be a link with the pollution."
The EA's investigations so far suggest a likely cause is contamination linked to pollution from the former tar works that operated in the area from the mid 19th Century up to the 1980s, but further work is needed.
Irvine said the issue had been even worse during this summer's heatwaves, which had led to her being prescribed medication for migraines.
"It would all bubble up, the smell would be so strong, it would surround the whole house," she said.
The EA said the pollution was a category three pollution incident because impacts were limited to the area where contamination enters the watercourse.
"I cant believe that even with all the health concerns and the fact there are otters in this river as well, it's still deemed minor," Irvine said.
In 2022, a leaked internal EA report told staff to ignore lower-impact category three and four pollution reports because of limited funding to investigate.
"I've been so frustrated with the EA," Irvine said. "It feels like pulling teeth trying to get any action from them."
The EA is also investigating another incident nearby at Brislington Brook, where dead fish and eels, foam on the water and "an intense chemical smell" were reported by volunteers.
Dominic Linnell, who walks his dog along the river every day, said he first noticed the issue more than two years ago.
"Two or three times a week, there's an awful chemical smell, and I see a huge oil slick that seems to bubble up from the bottom of the river," he said.
"I've seen fish floating dead and ducks. There's a lot of wildlife along there, you get herons, so it's a real shame.
"We all really care about the river, it's definitely killing wildlife and it's very unpleasant," he said.
Another local resident, Tristan Dhalla, said he had noticed the pollution ever since he moved to the area five years ago, and neighbours had even claimed the problem went back as far as a decade.
Wessex Water said their investigation had found "no issues" with any of their assets, while the substance did not pass through their sewerage network.
A Bristol City Council spokesperson said it was supporting the EA's investigation by providing information and records, while officers had visited the site last week.
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